Such a pressure sensor is known from EP-A2-0024 945 and serves for observing an absolute or relative pressure, especially in a motor vehicle engine, wherein a changed capacitance is detected by the bending of the semiconductor membrane. The counterelectrode of the known pressure sensor is applied as a conducting layer on an insulating glass substrate. The glass substrate, as a whole, is covered with a doped silicon layer in which apertures have been made by etching. The mid-region of the silicon layer, weakened by etching, operates as flexible membrane, while the unaffected rim of the silicon layer is bonded to the glass substrate. The silicon layer accordingly represents in its entirety one electrode of a measuring capacitor.
The manufacture of the known pressure sensor requires a series of technological steps of the thick film technology kind. For this reason, economical mass production is difficult to obtain. Since the entire silicon layer is constituted as an electrode, reference capacitances must be made available suplementarily outside of the pressure sensor. Finally, the known pressure sensor is unprotected with reference to disturbing voltages.
The invention has the object of providing a pressure sensor of the kind mentioned in the introduction, that can be manufactured with as few steps as possible of the now common and well developed silicon technology, and lends itself to providing reference capacitors and guard voltage electrodes in a simple way.
This object is accomplished in accordance with the invention by a applying a first conducting layer as a measuring electrode on the semiconductor membrane of a first semiconductor plate, being insulated from the latter, and applying a second conducting layer as the counterelectrode on a second semiconductor plate, insulated from the latter.
Since both electrodes have a semiconductor as substrate, similar readily controllable technological process steps are available for the manufacture of the first semiconductor plate carrying the semiconductor membrane and the second semiconductor plate carrying the counterelectrode.
In an advantageous embodiment of the invention, the semiconductor plates are silicon discs on which a silicon oxide layer has been produced by oxidation of the silicon to serve as an insulating layer. It is advantageous for both the measurement electrode and the counterelectrode to be formed as discs and to be surrounded by shield electrodes which are substantially ring-shaped. The shield electrodes can have a shielding voltage applied which has the same potential as the sensitive measurement electrode. In this manner disturbing electrical influences are excluded to a great extent. Particularly simple manufacture results if the silicon discs are electrically and mechanically bonded together by bumping technology.
Annular cavitites can be provided in the shield electrodes in order to provide room for the electrodes of a reference capacitor.
It is advantageous for the shield electrodes to be galvanically connected with the silicon plates, since then the entire silicon plate is effective as a shield electrode.
In an advantageous embodiment of the invention, a cover layer of silicon oxide as deposited by the decomposition of silane vapor is provided as the top layer of the silicon plates which acts as dielectric for the reference capacitance and as a spacer for the measurement capacitance.
Further advantageous embodiments are designated in the dependent claims.